1. Technical Field
The present invention generally relates to nucleating agent compositions and, more particularly, to a β-crystal form nucleating agent composition for polypropylene and use thereof.
2. Description of Related Art
Due to desirable tensile strength, compression strength, rigidity and folding resistance, polypropylene has been widely used in packaging containers, household appliances, automobile components and tubular products to substitute some engineering plastics, such as ABS. Moreover, polypropylene has small density and desirable processing and molding properties. However, compared with engineering plastics, polypropylene has lower heat distortion temperature and undesirable impact resistance, particularly low temperature impact resistance, which limits the further popularization of polypropylene. Therefore, in recent years, efforts have been focused on modification of polypropylene at home and abroad.
As known in the art, blend and multiphase copolymerization have been used to improve the impact properties of polypropylene. For instance, U.S. Pat. No. 3,562,790 disclosed a polymer blend having desirable impact strength and transparency. However, other properties of polypropylene, for instance rigidity, heat resistance property and processing performance are adversely affected. β-crystal form polypropylene belongs to hexagonal system and has desirable impact toughness and high heat distortion temperature, which can be used to overcome the disadvantages of copolymerized polypropylene. Appropriate melting temperature, crystallization temperature, temperature gradient, stress and nucleating agent are necessary conditions to obtain β-crystal form polypropylene resin, wherein adding β-crystal form nucleating agent into polypropylene is one of the simplest way. Adding β-crystal form nucleating agent to polypropylene can remarkably improve the crystallization temperature and crystallization rate of polypropylene and, subsequently, improve the content of β-crystal form polypropylene in polypropylene and improve the impact resistance of polypropylene.
DE-A-3610644, EP0682066, CN1004076B, EP0887475, CN1210103, CN1114651C and CN102181092A disclose various aspects of β-crystal form nucleating agents. However, none of these prior arts provides suggestion that adjuvant can be used to improve properties of the nucleating agent, or adjuvant(s) can be used to overcome the disadvantage in prior art that the flexural modulus of polypropylene will reduce when the amount of the β-crystal form nucleating agent being added increases.
Generally, there are two ways to add nucleating agent. One way is melt blending, as disclosed in CN 1944515A, wherein the formula containing β-crystal form nucleating agent is added to polypropylene resin and, then β-crystal form polypropylene composition is obtained via extrusion granulation. The other way is to add nucleating agent during polymerization process, as disclosed in CN 101429259A, wherein dried β-crystal form nucleating agent is added to a reaction kettle for bulk polymerization or for slurry polymerization. The bulk polymerization or slurry polymerization of propylene is catalyzed by Ziegler-Natta catalytic system to obtain composite powder of polypropylene and β-crystal form nucleating agent. The composite powder is heated or extruded via an extruding machine, to obtain β-crystal form polypropylene resin. Melt blending is widely used due to simple progress. However, the use amount of nucleating agent in melt blending is high and the components can hardly disperse evenly. Adding nucleating agent during polymerization process is very complicated and is rarely used in actual production.
The other disadvantage of using β-crystal form nucleating agent in prior art is although β-crystal form nucleating agent can improve the impact strength of the polypropylene, it will lead to reduction of one of the other key mechanical properties of polypropylene, i.e. flexural modulus. The rigidity of polypropylene will reduce when amount of β-crystal form nucleating agent added increases.